The present invention relates to ways or means of controlling flow of liquid refrigerant in a liquid vapor refrigeration system of the type having a compressor for pumping pressurized refrigerant through a condenser and through an expansion means for low pressure circulation through an evaporator where heat is absorbed to effect vaporization of the liquid refrigerant and cooling the surrounding air and for returning the vaporized refrigerant to the compressor inlet. The present invention relates particularly to refrigeration systems on board an automotive vehicle for air conditioning the vehicle passenger compartment. Such automotive passenger compartment air conditioning systems are known to employ a thermal expansion valve having a liquid filled chamber in heat transfer thermal relationship with a refrigerant flowing to return to the compressor such that changes in the sensed temperature of the refrigerant create expansion or contraction of the liquid in the chamber to thereby provide pressure forces for moving a diaphragm to control movement of the flow control valve.
Thermal expansion valves of the above-described type are thus mechanically self-contained and function independently of the electrical controls employed for energizing and deenergizing the clutch for coupling the compressor to the vehicle engine. Heretofore, the compressor clutch has typically been energized and deenergized by a pressure switch disposed to sense the saturation pressure at the pressure discharge side of the evaporator or in the line between the expansion valve and the evaporator inlet. However, it has been desired to eliminate the pressure switch as a source of service problems particularly leakage in the switch or its connections to the line. It has further been desired to eliminate the pressure switch in view of the cost of providing a pressure switch having sufficient proof pressure rating yet sufficient accuracy for cycling the compressor in response to small changes in saturation pressure.
It has therefore been desired to employ a mechanical thermal expansion valve to benefit from the low cost and proven reliability thereof, and to provide an alternative to a pressure switch for cycling the compressor clutch in an automotive air conditioning system. It has further been desired to find a way or means of generating an electric signal in response to small changes of the saturation pressure in an automotive air conditioning system evaporator inlet line in a manner which would enable the use of a microprocessor to benefit from the sophistication of the control available from a microprocessor.
It has also been desired to provide a way or means of controlling an electrically operated condenser cooling fan in response to upper and lower limit conditions of refrigerant pressure and yet retain the use of a mechanical thermal expansion valve.
It is known for example as taught in U.S. Pat. No. 4,841,734, commonly owned with the present invention, to utilize a "self-heated" thermistor having a controlled current flow therethrough and to sense the voltage drop across a resistor in series with the thermistor and to convert the voltage drop to saturation pressure from a look-up table of known refrigerant properties in a refrigeration system and particularly an automotive air conditioning system. In the aforesaid patent the saturation pressure signal from a microprocessor is used to deenergize the compressor when excessive saturation pressure is detected. The self-heated thermistor is disposed on the pressure line near the inlet to the expansion valve means.
It is also known as taught for example in U.S. Pat. No. 4,835,976, commonly owned with the present application, to employ a self-heated thermistor in the suction line between the evaporator and the compressor inlet to determine the quality of the refrigerant flowing thereover for enabling a microprocessor to generate a control signal for pulsing a solenoid to operate an electric expansion valve.
It is known also, as taught in U.S. Pat. No. 4,794,762 to provide a thermistor at the inlet and outlet of the mechanical expansion means for providing a refrigerant temperature signal to a microcomputer from the high and low pressure side for comparison with high and low limits for fan and compressor clutch cutout.